This invention relates to the field of printing, and in particular to a method and apparatus for mounting flexographic or letterpress plate segments onto a printing plate carrier.
Flexography and letterpress use printing cylinders on which printing plates are mounted using different mounting methods. When printing in color, one cylinder is used for each color, i.e., for each color separation.
Conventional printing uses a single printing plate on the cylinder covering the whole area to be printed. An alternate is using a plurality of segments of plate material mounted on the cylinder. Such a method uses less printing plate material than using a single plate covering the whole area to be printed, and is suitable for flexography and letterpress because flexographic and letterpress plate material is relatively expensive. An economical method for creating plate segments, each containing register marks is described in U.S. patent application Ser. No. 09/946,145 to Klein, et al., titled “METHOD, APPARATUS, AND COMPUTER PROGRAM FOR REDUCING PLATE MATERIAL WASTE IN FLEXOGRAPHY.”
A known manual method for mounting the plate segments onto a carrier to produce a cylinder with multiple plate segments includes first producing flat printing plate sheets, cutting the plate segments, and manually mounting the segments onto a printing plate carrier such as a drum, sleeve, or a mylar sheet. Several tools exist to facilitate this operation, e.g., so that the operation is more repeatable. Success is highly dependent on the skill of the operator.
The manual method includes applying glue to the back of the printing plate segments or making the printing plate carrier adhesive, e.g., by applying glue or by using double sided adhesive tape. The manual method further includes the operator manually aligning register marks on the printing plate segments with marks that have been provided by a mounting apparatus. The operator can use a prior-art mounting apparatus for this. The apparatus helps registering by using a half-transparent mirror or a video screen to display a register mark on top of the image of the plate segments. In all such procedures, an operator manually decides where to mount the plate segment. As a result, the quality of the resulting printing cylinders may vary depending on the skill of the operator. Furthermore, the mounting may take a relatively long time, especially when high precision is required. Furthermore, large plate segments are more difficult to mount than smaller plate segments, mainly because it is difficult for human operators to handle large plate segments.
An apparatus that help a human operator to mount printing plate segments onto a printing cylinder is called a manual mounting machines herein, and the mounting method is called a manual mounting method.
Another prior art method for producing cylinders that include printing plate segments produces the printing cylinders directly. Blank, unimaged plate segments are applied on a cylindrical plate carrier, typically a sleeve, and imaged in a computer-to-plate drum imaging device such as the Esko-Graphics Cyrel Digital Imager (Esko-Graphics NV, Gent, Belgium, the assignee of the present invention). The imaging device is used to directly expose the sleeve carrying the flexographic plate segments. After exposure, the cylindrical plate carrier with the exposed plate segments attached is moved away from the imaging device and processed in round washing equipment. Because the printing plate segments are not removed from the sleeve or printing cylinder for the processing, the image register is maintained throughout the process until printing. The advantage of this method over the more conventional method of manually mounting imaged segments is the improved register and decreased mounting cost. The second method however is less popular than the manual mounting method, mostly because of cost. Producing imaged flat flexographic plate sheets for is relatively inexpensive because the equipment is widely available and its cost mostly amortized. There is substantial investment required for exposing flexographic plates “in the round,” i.e., on blank segments mounted on cylindrical carriers.
One of the problems with using a plurality of segments is accurate registration. As a result, imaging on a single sheet is still often used in flexography, even at the cost of the wasted plate material.
Thus there is a need in the art for an apparatus and method of producing print-ready cylinders by first exposing flat plate materials and cutting the flat plates into segments, while maintaining register accuracy that does not depend on the skill of the operator to the same extent as the known manual method. There further is a need in the art for a mounting device that provides for mounting exposed plate segments onto a plate carrier while maintaining accuracy, such a device being considerably less expensive than a full “in the round” processing unit.